1. Field of the Invention
This invention relates to an optically anisotropic device and production method thereof. More particularly, the present invention relates to a biaxial optical anisotropy device which will be suitable for compensating for optical anisotropy of a liquid crystal cell of a liquid crystal display (LCD) device and production method thereof.
2. Description of the Related Art
A homeotropic orientation type liquid crystal display device has been utilized as a kind of liquid crystal display device. The structure and characteristics of this device will be explained with reference to FIGS. 6, 7 and 8 of the accompanying drawings.
FIG. 6 is an explanatory view for explaining the principle of display in one pixel of a conventional homeotropic orientation type liquid crystal display device. In the drawing, -z direction is a light transmission direction, and x and y axes cross orthogonally each other and also cross orthogonally the z axis. Polarizers 10 and 40 are disposed in parallel with each other in such a manner that their polarization axes (represented by arrows A and B) cross orthogonally each other and form an angle of 45.degree. with the x and y axes. A homeotropic orientation type liquid crystal cell 20 is sandwiched between these polarizers 10 and 40. The liquid crystal cell 20 comprises transparent glass substrates 21, 23 equipped with transparent electrodes and disposed in parallel with each other, and a liquid crystal layer 22 sandwiched between the substrates 21 and 23.
The liquid crystal has a tilt angle which is slightly inclined in the x direction from the z axis. Molecules 25 of the liquid crystal 22 are oriented substantially perpendicularly to the substrates under the state where no voltage is applied. The light which is polarized in the direction A in the polarizer 10 does not receive any action in the liquid crystal layer 22 and is incident on the other polarizer 40. The transmitted light is blocked because the polarizing direction B of the polarizer 40 crosses orthogonally the polarizing direction A of the polarizer 10. For this reason, when the pixel is viewed from the z axis direction of the liquid crystal display device, it appears dark.
Under the voltage application state, the liquid crystal molecules 25 exhibit a greater tilt angle particularly at the center portion of the liquid crystal layer. The liquid crystal layer exhibits birefringence under this state. The light transmitting through the polarizer 10 receives the action of birefringence when it passes through the liquid crystal layer 22 and is incident into the polarizer 40. The action of birefringence becomes maximal when the polarization axis A makes 45.degree. with the x axis along which the tilt angle exists. Therefore, among the rays of light incident into the polarizer 40, a part of them passes through the polarizer 40, and the pixel becomes bright when viewed from the z axis direction of the liquid crystal display device.
The explanation given above deals with the case where the light travels along the z axis. When an operator of the liquid crystal display device moves away from the z axis, the liquid crystal cannot be handled as an isotropic medium even under the OFF state. In other words, the birefringence cannot be neglected even under the OFF state and the leakage of light occurs. For this reason, a viewing angle is limited.
A uniaxial optical compensation cell 30 is employed for compensating for the optical anisotropy of a liquid crystal layer so that satisfactory display can be made over a wide range of viewing angle deviated from the z axis by improving the viewing angle. The optical properties of the liquid crystal layer 22 and the optical compensation cell 30 with the refractive indices nx, ny, nz and n'x, n'y, n'z in the axial directions x, y and z, respectively, have the following relations: ##EQU1## In other words, the refractive index distribution of the liquid crystal layer 22 is like a Rugby football which is elongated in the z axis direction as shown on the lefthand side of FIG. 6, while the refractive index distribution of the optical compensation cell 30 is like a tablet which is short in the z axis direction, contrary to the liquid crystal layer 22. A liquid crystal display device having a satisfactory viewing angle can be obtained by adjusting appropriately retardations of the liquid crystal layer 22 and the optical compensation cell 30.
In a dot matrix liquid crystal display device using the conventional optical compensation cell as described above, a very small voltage (OFF voltage) is applied even when the device does not achieve display. Therefore, the direction of orientation of the molecules 25 in the liquid crystal layer 22 are inclined with a small tilt angle relative to the z axis, so that the leak of light occurs even under the OFF state and hence black level is deteriorated. In other words, a display contrast drops and display quality is deteriorated
FIG. 7 shows transmission vs. voltage characteristics of a conventional liquid crystal display device. In the conventional device, the transmission does not become zero even under the OFF voltage (V.sub.OFF) state.
FIG. 8 shows the change of the input voltage of the conventional liquid crystal display device and that of the transmission with time. Due to the leak of light T.sub.OFF at the time of OFF voltage (V.sub.OFF), rise speed (T.sub.OFF -90% T.sub.ON =150 msec) is slower than fall speed (T.sub.ON -10% T.sub.OFF =100 msec)
Furthermore, in such a conventional liquid crystal display device, reflection of light occurs on the glass interface when the viewing angle is high, and the propagation mode of light varies, thereby causing asymmetry that the viewing angle is different on the right and left.